WO2017046653A1 - Method and apparatus for the direct reduction of iron ores utilizing coal-derived gas or syngas, with improved energy efficiency - Google Patents

Method and apparatus for the direct reduction of iron ores utilizing coal-derived gas or syngas, with improved energy efficiency Download PDF

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Publication number
WO2017046653A1
WO2017046653A1 PCT/IB2016/001444 IB2016001444W WO2017046653A1 WO 2017046653 A1 WO2017046653 A1 WO 2017046653A1 IB 2016001444 W IB2016001444 W IB 2016001444W WO 2017046653 A1 WO2017046653 A1 WO 2017046653A1
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Prior art keywords
gas
water
stream
humidifier
reduction
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PCT/IB2016/001444
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French (fr)
Inventor
Eugenio Zendejas-Martinez
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Hyl Technologies, S.A. De C.V.
Danieli & C. Officine Meccaniche S.P.A.
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Publication of WO2017046653A1 publication Critical patent/WO2017046653A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B5/00—Making pig-iron in the blast furnace
    • C21B5/06—Making pig-iron in the blast furnace using top gas in the blast furnace process
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/0073—Selection or treatment of the reducing gases
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B7/00—Blast furnaces
    • C21B7/002—Evacuating and treating of exhaust gases
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
    • C21C5/28—Manufacture of steel in the converter
    • C21C5/38—Removal of waste gases or dust
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B5/00—Making pig-iron in the blast furnace
    • C21B5/001—Injecting additional fuel or reducing agents
    • C21B2005/005—Selection or treatment of the reducing gases
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B2100/00—Handling of exhaust gases produced during the manufacture of iron or steel
    • C21B2100/20—Increasing the gas reduction potential of recycled exhaust gases
    • C21B2100/24—Increasing the gas reduction potential of recycled exhaust gases by shift reactions
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B2100/00—Handling of exhaust gases produced during the manufacture of iron or steel
    • C21B2100/20—Increasing the gas reduction potential of recycled exhaust gases
    • C21B2100/26—Increasing the gas reduction potential of recycled exhaust gases by adding additional fuel in recirculation pipes
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B2100/00—Handling of exhaust gases produced during the manufacture of iron or steel
    • C21B2100/20—Increasing the gas reduction potential of recycled exhaust gases
    • C21B2100/28—Increasing the gas reduction potential of recycled exhaust gases by separation
    • C21B2100/282—Increasing the gas reduction potential of recycled exhaust gases by separation of carbon dioxide
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B2100/00—Handling of exhaust gases produced during the manufacture of iron or steel
    • C21B2100/60—Process control or energy utilisation in the manufacture of iron or steel
    • C21B2100/62—Energy conversion other than by heat exchange, e.g. by use of exhaust gas in energy production
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C2100/00—Exhaust gas
    • C21C2100/02—Treatment of the exhaust gas
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C2100/00—Exhaust gas
    • C21C2100/04—Recirculation of the exhaust gas
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C2100/00—Exhaust gas
    • C21C2100/06—Energy from waste gas used in other processes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/10—Reduction of greenhouse gas [GHG] emissions
    • Y02P10/122—Reduction of greenhouse gas [GHG] emissions by capturing or storing CO2
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/10—Reduction of greenhouse gas [GHG] emissions
    • Y02P10/134—Reduction of greenhouse gas [GHG] emissions by avoiding CO2, e.g. using hydrogen
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • the invention relates to the direct reduction of iron ores in a reduction system comprising a direct reduction process for producing direct-reduced-iron (DRI) utilizing a coal-derived gas or a syngas having a high CO content and a low ratio of H 2 /CO, wherein the gas composition is modified by CO conversion to H 2 and C0 2 followed by H 2 0 and C0 2 removal (using the water- gas-shift reaction).
  • the raw coal-derived gas may be produced by a variety of processes such as by partial oxidation, pyro lysis or gasification of hydrocarbons, for example coal gasifiers, coal melter-gasifiers, coke ovens, or even partial oxidation of gaseous or liquid fuels and the like (syngas).
  • the invention relates to a process and system to produce DRI with an improved energy efficiency and achieves this because more heat of the hot top gas effluent from the reduction reactor is recuperated by means of a liquid water circuit, as compared to using the heat only for steam generation.
  • liquid water is heated by said hot gas withdrawn from the reduction reactor and the heated water is contacted with the coal-derived gas or syngas to provide all the H 2 0 necessary for the CO conversion reaction.
  • Direct reduction of iron ores for producing pre-reduced metallized materials useful for the production of steel is becoming more and more widespread in the steel industry.
  • Some of the advantages of direct reduction plants are that the production capacity may be relatively small as compared with pig iron production in coke-fed blast furnaces and that the metallic iron is produced in solid form with low sulfur and silicon content and that the DRI may be easily melted in electric-arc furnaces.
  • the reducing agents utilized in the direct reduction plants are hydrogen and carbon monoxide, most typically produced by reformation of natural gas and therefore, these plants have been built in areas where natural gas is available and at relatively low price.
  • Hydrogen and carbon monoxide can also be produced by a variety of processes as partial oxidation, pyrolysis or gasification of hydrocarbons, for example coal gasifiers, coal melter- gasifiers, coke ovens, partial oxidation of gaseous or liquid fuels and the like.
  • Coal-derived gas or syngas however typically has a low ratio of H 2 /CO as measured in volume, and therefore a high proportion of CO as compared with the traditional reformed gas produced by catalytic reformation of natural gas with H 2 0 and C0 2 .
  • the water necessary for the reaction is typically added to the raw syngas as steam.
  • the steam can be produced in a dedicated boiler using a suitable fuel to produce the required amount of steam as described in the following prior-art patent publications:
  • US Patent No. 5,882,579 to Viramontes-Brown et al. describes a method and apparatus for producing DRI using syngas produced in a melter-gasifier and two reduction reactors.
  • the gas effluent from the first reduction reactor associated to the melter-gasifier is treated to modify its CO content in at least one catalytic reactor (shifter) and the C0 2 resulting from the shift reaction and also the C0 2 from the reduction reactions in said second reduction reactor is removed from a combined stream of shifted-gas and recycled top gas effluent from the second reduction reactor.
  • the water needed for the CO conversion is produced utilizing the heat of the top gas effluent from the second reactor to produce steam.
  • US Patent No. 8,709,131 to Meissner et al. describes a method and system to produce DRI using synthesis gas, which may be generated in coal gasification processes, wherein the top gas withdrawn from the reduction reactor is treated in a CO shift catalytic reactor and then it is combined with raw synthesis gas before C0 2 is removed from the combined gas stream. The resulting gas stream is heated in two steps: a first step in a convection heater and a second step by partial combustion with oxygen, and is then fed to the reduction reactor.
  • This patent does not teach or suggest using the heat of the top gas to provide the water necessary for the CO shift reaction.
  • This patent teaches using steam from an external source of the system and therefore the energy efficiency of the reduction system is lower as in the present invention.
  • the heat of the reducing gas effluent from the reduction reactor is not used but wasted.
  • US Patent No. 5,958,107 to Greenwalt discloses a combination of a melter-gasifier unit 12 where the excess reducing gas effluent from a first reduction reactor 10 is subject to a CO conversion step to modify the content of carbon monoxide in said reducing gas before it is further utilized in a second direct reduction reactor 44 to produce DRI.
  • This patent teaches that the water necessary to carry out the shift reaction of CO is provided in two ways: 50% by saturation of the gas stream in a saturator 24 where liquid water is sprayed over the reducing gas; and 50% by feeding steam 26 to the shift reactor.
  • the steam may be produced using water condensed from a gas cooler after the shift reaction in a boiler 34.
  • the heat to produce the steam 26 is an additional energy input to the plant.
  • the process of this patent is not as thermally efficient as the invention because it requires producing steam and consequently needs additional energy from a source external to the system.
  • the present invention uses as much heat as possible of the hot gas effluent from the reduction reactor, in the form of heated liquid water, to saturate the gas fed to the CO shift reactor and thus provide all the water necessary for the CO conversion. This is possible because the heat transfer coefficient between liquid water and a gas is higher than between two gas streams.
  • the process of Greenwalt is also inefficient in the use of the reducing gases because the top gas effluent from the reduction reactor 44 is not upgraded by removal of C0 2 which is a product of the reduction reactions, therefore, the amount of top gas that can be recycled is limited and a good portion of the top gas has to be used as fuel to get rid of the C0 2 generated in the reactor 44.
  • the present invention overcomes this disadvantage by combining the recycled gas from the reduction reactor with the gas "shifted" make-up gas stream and the resulting combined gas stream passes through the C0 2 removal unit thus allowing recycling almost all available reducing gases present in the top gas effluent from the reduction reactor.
  • U.S. Patent Application No. 2014/0202285 Al describes a method and system for treating waste gases from plants for pig iron production where said gases are utilized in a direct reduction plant.
  • This patent application teaches a method of obtaining a gas stream with a desired ratio H2/CO between 1.5 to 4.5 starting from a syngas stream having a low ratio 3 ⁇ 4/CO for example of 0.8, by mixing two gas streams: a first stream which has been subjected to CO conversion and C0 2 removal having a high ratio H 2 /CO for example of 16 and a second stream not having been subjected to CO conversion, only having been stripped of C0 2 and therefore having the same low ratio H 2 /CO for example of 0.8 of the untreated initial gas stream.
  • This patent application does not teach or suggest establishing a liquid water circuit for heat recuperation from the top gas effluent from the reduction reactor with higher efficiency. The heat content of the top gas 46 exiting the reduction reactor 18 is not used but wasted in cooler 49.
  • This patent application leads away of using liquid water for supplying all the water for the CO conversion and teaches that steam may be produced from other hot gases in the pig-iron production plant.
  • the system of this patent application also has the disadvantage of requiring two separate C0 2 separation units in order to produce two gas streams with different composition which will be mixed to obtain the desired composition for iron ore reduction.
  • the system of this publication has another drawback related to the introduction of a portion of the gas stream with high CO content into the transition zone of the reduction reactor with the purpose of increasing the carbon content of the DRI. It is known that carbon deposition from CO is not favored at high temperatures and at low temperatures so the carbon content in DRI is relatively low.
  • the carbon content in DRI can be controlled in a wide range from about 2 % to about 4% by regulating the concentration of hydrocarbons in the reduction reactor.
  • the carbon deposition is achieved by hydrocarbons cracking which is more effective at the process conditions of the transition zone or the cooling zone of the reduction reactor.
  • the heat content of the reducing gas effluent from the reduction reactor is used to increase the water content of the reducing gas prior to the shift reaction of CO by means of a liquid water circulation system, offering a number of previously unrecognized advantages as, for example, increasing the amount of recovered heat from the hot top gas effluent from a reduction reactor and not requiring high-quality water for steam generation.
  • the objects of the invention are generally achieved by providing a method for producing DRI utilizing a synthesis gas containing a relatively high content of CO, with a ratio H 2 /CO lower than about 0.5, in a reduction system comprising a reduction reactor from which a hot stream of reducing gas is withdrawn as a top gas, a heat-exchanger wherein heat is taken from said hot top gas and transferred to a stream of liquid water; and a gas humidifier.
  • the objects of the invention can also be more particularly achieved by providing a method method for producing DRI with improved energy efficiency in a direct reduction plant wherein a make-up first gas stream of at least one of a coal-derived gas or a syngas or a top gas from a reduction reactor or an export gas from a metallurgical furnace is subjected to a water- gas-shift reaction to H 2 and C0 2 by reacting CO with H 2 0 in at least one catalytic reactor resulting in a shifted reducing gas stream, which is used as reducing gas in a direct reduction reactor; said process being characterized by withdrawing a hot second gas stream from said reduction reactor; passing said second hot gas stream through a heat exchanger wherein heat is transferred from said second hot gas stream to a liquid water stream resulting in a heated liquid water stream; contacting at least a portion of said first gas stream with said heated liquid water stream in a humidifier to increase the water content of said first gas stream, thus providing all the H 2 0 necessary for said water-gas-shift reaction, resulting
  • the objects of the invention are also generally achieved by providing an apparatus for producing DRI utilizing a synthesis gas containing a high relative content of CO, with a ratio H2/CO lower than about 1, in a reduction system
  • a reduction reactor having a reduction zone, a gas-water heat exchanger; first gas conduit means connecting the heat exchanger to said reduction zone; a water gas humidifier where at least a portion of a top gas stream effluent from said reduction zone is contacted with liquid water that has been heated in said heat exchanger with heat of said top gas stream effluent from said reduction zone; second gas conduit means connecting the heat exchanger with said gas humidifier; at least one CO conversion reactor; third gas conduit means connecting the gas humidifier with said CO conversion reactor; a C02 removal unit; and fourth gas conduit means connecting said CO conversion reactor with said C02 separation unit; liquid water conduit means connecting said heat exchanger with said gas humidifier to circulate heated liquid water from the heat exchanger to said humidifier and returning liquid water from said humidifier to the heat exchanger after having contacted said top gas stream eff
  • Figure 1 is a schematic process diagram of a direct reduction plant using a coal-derived gas or a syngas wherein the heat of the hot top gas effluent from the reduction reactor is recuperated in a gas-liquid heat exchanger by a liquid water circuit and the heated water is contacted with a stream of the coal-derived gas or syngas; whereby all the water needed to modify the composition of said coal-derived gas in a CO conversion step is supplied by saturation of said gas stream.
  • Figure 2 is a schematic process diagram of the combination of a melter-gasifier coal gasification plant and a direct reduction plant incorporating the liquid water circuit for heat recuperation, wherein said melter-gasifier plant comprises an associated direct reduction reactor and the excess gas exported from said associated direct reduction reactor is used to produce DRI in said direct reduction plant.
  • Figure 3 is a schematic process diagram of the combination of a melter-gasifier coal gasification plant and a direct reduction plant incorporating the liquid water circuit for heat recuperation, wherein hot syngas produced in the melter-gasifier plant is fed directly to the reduction reactor of said direct reduction plant, combined with hot recycled and upgraded reducing gas, without passing first through an associated direct reduction reactor.
  • a reduction plant 10 comprises a reduction reactor 12 having an upper reduction zone 14 and a lower discharge zone 16.
  • Particulate solid iron ores 18 in the form of lumps or pellets are contacted in the reduction zone 14 with a high-temperature reducing gas from pipe 20 comprising hydrogen and carbon monoxide and thereby producing direct reduced iron (DRI) 22.
  • the DRI is discharged from said reactor 12 through the lower discharge zone 16. Depending on the type of subsequent utilization of the DRI, it may be discharged hot or cold.
  • the lower discharge zone 16 of reactor 12 may optionally have means (not shown), well known in the art, for circulating a stream of cooling gas for cooling down said DRI to a temperature level below about 100°C before its discharge from said reactor. See for example the cooling/discharge zone 14 with an associated cooling gas loop shown in US patent no. 4,524, 030.
  • a stream 15 of a hydrocarbon-containing gas for example methane, such as coke oven gas, natural gas or liquified petroleum gas (LPG), can advantageously be used for increasing the carbon content of the DRI.
  • methane such as coke oven gas, natural gas or liquified petroleum gas (LPG)
  • LPG liquified petroleum gas
  • the hydrocarbon-containing gas 15 is injected into the transition zone of the reactor 12, below the reduction zone 14 so that the high temperature of the DRI flowing down from the reduction zone cracks the hydrocarbons into carbon and hydrogen, resulting in DRI with higher carbon content.
  • the carbon content of the DRI can be controlled by regulating the amount of the hydrocarbon-containing gas introduced into the reduction reactor. If the reduction reactor is designed to produce cold DRI, then the hydrocarbon-containing gas can also be injected to the cooling gas circuit (not shown for simplicity) and the carbon content of the DRI can also be controlled by regulating the amount of hydrocarbon-containing gas fed to the cooling gas circuit.
  • Spent reducing gas exits from the reduction zone 14 at a temperature in the range from about 300°C to about 500°C via pipe 24, identified here as a second gas source, to be used for upgrading in a recycle circuit and return back to the reduction zone 14.
  • Such recycle reducing gas initially passes through a heat exchanger 26 where its sensible heat is used to preheat a stream of liquid water which is used in a gas humidifier 30.
  • the spent reducing gas effluent from the reduction zone 14 contains significant amounts of water and carbon dioxide produced as by-products from the reactions of hydrogen and carbon monoxide with the iron oxide content of the iron ore 18.
  • the upgrading of the reducing gas effluent from reduction zone 14 begins in the cooler/scrubber 34, where the water produced by the hydrogen reduction reaction condenses and is extracted from the system through pipe 38 along with the cooling water 36.
  • a minor portion of the cleaned and dewatered spent gas is purged from the recycle circuit through pipe 40 having a pressure control valve 42, for pressure control of, and for maintaining a lowN 2 concentration in the recycle circuit.
  • the purged gas may be advantageously utilized as fuel in the burner (not shown) of the gas heater 58 and optionally, if needed, may also be supplemented with some suitable fuel gas.
  • the remaining portion of the cleaned and dewatered reducing effluent gas is then transferred to compressor 48 through pipes 50 and 54, wherein its pressure is raised to a level suitable for its ultimate recycling to the reduction zone 14 of the reactor 12.
  • the recycled reducing gas stream 50 after being combined with make-up gas stream 52 of upgraded syngas forms the reducing gas stream 53 which, after compression by compressor 48 is treated in a C0 2 removal unit 56.
  • the C0 2 removal system is of the chemical type using a liquid solution, typically of diethanolamines, to absorb the C0 2 , then C0 2 57 is withdrawn from the reduction system.
  • C0 2 removal system is of the physical type using adsorbent materials (PSA or VPSA), or gas membranes, to adsorb the C0 2 , then C0 2 57 is withdrawn from the reduction system mixed with other gases like hydrogen, CO and methane, which can be utilized as fuel in the gas heater 58.
  • PSA adsorbent materials
  • VPSA adsorbent materials
  • gas membranes gas membranes
  • the C0 2 -lean reducing gas stream exiting from C0 2 removal unit 56 passes through pipe 60 to heater 58 where its temperature is raised to above 850°C, preferably above 950°C.
  • the hot reducing gas exiting from heater 58 through pipe 62 may be thereafter combined with oxygen, or oxygen enriched air, injected through pipe 64 from source 66, to further raise its temperature in the range from 1000°C to 1050°C.
  • the hot reducing gas flows then through pipe 20 and is fed to the reduction zone 14 of reduction reactor 12.
  • Coal-derived gas or syngas from a make-up gas source 68 having a high content of CO produced for example from a coal gasifier or a melter-gasifier system or a coke plant, which has been cooled/washed and compressed to a pressure sufficient to be injected into the reduction system, flows as a first gas stream through pipe 70 and pipe 72 to a humidifier 30 where the gas stream is saturated with water.
  • the saturated make-up gas stream flows from humidifier 30 through pipes 96 and 82 to a first CO-conversion vessel 84 containing a bed of catalyst 86. Since the CO conversion reaction is exothermic, the syngas stream is passed through heat exchanger 88 to bring down its temperature to the order of 300°C to 350°C and is then passed through a second CO-conversion vessel 90 containing a second bed of catalyst 92 to increase the efficiency of the CO-conversion to H 2 and C0 2 .
  • the syngas make-up stream exiting the second CO-conversion vessel 90 flows through pipe 94 and is cooled down in heat-exchanger 80 wherein the heat developed by the exothermic reaction in catalytic vessel 90 is used to pre-heat the water-saturated gas stream 96 effluent from humidifier 30.
  • a pipe 98 and flow control valve 100 provide a by-pass line for adjusting the water content of the syngas to the desired level to carry out the CO conversion, by means of a temperature sensor and a pressure sensor 102 which emits signals indicative of the values of temperature and pressure of the gas stream in pipe 96. These signals are used to determine the water content of the syngas exiting the humidifier 30.
  • a suitable automatic controller uses the calculated value of water content of the gas stream in pipe 96 to emit a control signal to flow control valve 100 so as to increase or decrease the flow of gas bypassing humidifier 30 and in this way allow less or more gas in contact with the heated liquid water. Since the amount of heat recovered in heat exchanger 26 is substantially constant, then the water-content in the gas stream exiting the humidifier 30 can be regulated at the level necessary to efficiently carry out the CO- conversion reaction by allowing more or less gas pass through humidifier 30.
  • a stream of oxygen or oxygen-enriched air from source 66 is optionally added via pipe 64 to the hot reducing gas stream which further increases its temperature and therefore increases the productivity of the reduction reactor.
  • the reducing gas entering the reduction zone 14 through pipe 20 preferably has a composition characterized by a ratio of H 2 /CO in the range from 1.5 to 4.0 in volume percent and a reduction potential (H 2 + CO) / (3 ⁇ 4 + CO + H 2 0 + C0 2 ) in the range of 0.8 to close to 1.
  • a circuit of liquid water is established, circulating through heat-exchanger 26 and humidifier 30, by means of pipes 74, 76 and a water pump 78.
  • This liquid water circuit allows for a more efficient heat recovery from the reducing gas effluent from the reduction zone 14 because the liquid water may take heat at lower temperature as compared to the heat exchangers used for steam production.
  • Water from source 110 flows through pipe 112 and is pre-heated in heat exchanger 114, exiting through pipe 116 and passing through another heat exchanger 88 where its temperature is raised to a range between 150°C and 180°C and then is used as make-up water adding it through pipe 118 to the liquid water circuit 120 comprising heat exchanger 26, humidifier 30, pipes 74 and 76 and pump 78.
  • a portion of the hot water circulating through the hot water circuit is withdrawn through pipe 128 provided with flow control valve 130 to purge some water (through pipe 132) to minimize concentration of sediments and dirt in the water circuit 120.
  • This blow-down hot water stream is passed through heat exchanger 114 and contributes to preheat the make-up water stream from pipe 112.
  • a pipe 122 with a flow control valve 124 is provided to by-pass a controlled amount of hot liquid water in response to a control signal of a temperature sensor 126 measuring the temperature of the water returning to heat exchanger 26 through pipe 74.
  • This control loop of the temperature of the liquid water returning to heat exchanger 26 is used to prevent the water entering heat exchanger 26 from reaching too low a temperature that may cause condensation of the water contained in the top gas stream and cause problems of clogging and gas flow obstructions in heat exchanger 26.
  • the temperature of the water stream entering heat exchanger 26 is maintained above 120°C at a gas pressure of about 5 Kg/cm2A.
  • the condensation temperature of water of course depends on the pressure of the gas and therefore the range of temperature for this control has to be adapted to the particular operation conditions of each application.
  • the valve 124 allows more hot water bypass the humidifier 30 raising in this way the water temperature.
  • Using a liquid water circuit for heat recuperation as in the present invention provides the advantage of using about 30% more heat as compared with the currently used steam production heat exchangers.
  • the amount of heat recuperated when using a liquid water circuit is about 0.27 Gcal/ton of DRI while only about 0.21 Gcal/ton of DRI can be recuperated using a steam production heat exchanger.
  • Another advantage of using liquid water instead of producing steam in heat-exchanger 26 is that the quality of water used is less demanding, thus providing operational and capital savings.
  • FIG. 2 a schematic diagram of the liquid water heat recuperation circuit 120 is shown as applied to a combination of a melter-gasifier plant having a melter-gasifier 134, wherein fossil fuels such as coal, refinery residues and biomass are partially oxidized with oxygen 136 and steam 138 in a manner known in the art.
  • the syngas exits through pipe 140 is de-dusted and partially cleaned in a hot cyclone 142, and is then fed to a reduction reactor 144, to which iron ore 146 is also fed to produce a metallized product which is discharged through a discharge system 148 and fed to the melter-gasifier 134, where the product is melted.
  • Pig iron 150 and slag 152 are tapped from melter-gasifier 134 as known in the art.
  • a schematic diagram of the liquid water heat recuperation circuit 120 is shown as applied to a combination of a melter-gasifier plant having a melter- gasifier 134, wherein the hot syngas exiting through pipe 70, after de-dusting and partial cleaning in a hot cyclone 142, is directly fed to the reduction zone 14 combined with upgraded recycled gas effluent from the reduction zone 14.
  • the hot syngas stream 70 is at a high temperature (above 900°C) and is combined with hot recycled and upgraded gas which has been subjected to the CO conversion step as described in connection with Figure 1 to obtain the required composition about 50% of the reducing gas required in reduction zone 14.
  • the rest of the process is the same as described in connection with Figure 1.
  • the process of the invention offers a number of advantages over the prior art. For example, it does not require steam production spending additional thermal energy for supplying the water used as reactant in the catalytic shift reactor.
  • the hot water necessary for gas saturation is heated using waste energy from the gas effluent from the reduction reactor instead of being produced in external heaters or boilers.
  • the energy efficiency of the DR process is particularly improved, since about 0.27 Gcal per ton of DRI is recuperated from the hot gas effluent from the reduction reactor as compared with other prior-art processes that produce steam with the heat of said effluent gas stream or require an additional boiler.
  • the higher thermal efficiency also comes from the fact that the heat transfer coefficient for a gas-liquid water heat exchanger is higher than for gas-gas heat exchanger.
  • the temperature of the gas stream is lower as compared with the lowest temperature that can be reached for the gas stream and still produce steam.
  • the water circuit allows recuperation of almost all the heat available, because all the non- evaporated excess water withdrawn from the gas humidifier is re-heated in the heat exchanger without any heat waste.
  • a process calculation for a DRI plant having a production capacity of 220 metric tons of DRI per hour indicates the following:
  • a second gas stream of hot top gas 24 exits the reduction zone 14 of the DRI plant at a temperature of 484°C, a flow rate of 463,289 NCMH and a water content of 22.2% volume.
  • This gas stream passes through a heat exchanger 26 (See figure 1) and exits through pipe 32 at a temperature of 130°C, whereby the major part of its sensible heat is transferred to a stream of liquid water 74 entering said heat exchanger at a temperature of 115°C and is heated to form a heated liquid water stream 76 at a temperature of 165°C.
  • the shift modified resulting fourth gas stream 94 is then eventually recycled back to the reduction zone 14, after being subsequently cooled down in cooler 108 whereby water is condensed and the water content in the gas stream is lowered from 18.4%) to 0.8%) and then being combined with the cooled top gas effluent 50 derived from the reduction zone 14.
  • the combined gas stream 53 passes through a compressor 48 and is fed to a PSA - C0 2 separation unit 56 resulting in a reducing gas stream in pipe 60 having a high reduction potential measured as (H 2 + CO) / (3 ⁇ 4 + CO + H 2 0 + C0 2 ) of 0.9 adequate for carrying out the reduction reactions in the reduction zone 14 and a ratio H 2 / CO of 3.4 adequate for its handling in the reduction system without problems in the gas heater.

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Abstract

A process for producing DRI with improved efficiency in a direct reduction plant comprising a reduction reactor, a heat exchanger, a gas humidifier, and a CO2 removal unit, wherein a coal-derived gas having a high content of CO is subject to CO conversion with H2O to H2 and CO2 resulting after passing through the CO2 removal unit in a reducing gas with an adequate composition for its use in the reduction reactor. A liquid water circuit is established to heat water in said heat exchanger using the heat content of the hot gas stream withdrawn from the reduction reactor and to evaporate hot water in the humidifier, for saturating at least a portion of the coal-derived gas stream with H2O by contact with said heated liquid water stream, thereby supplying all the H2O necessary for the CO-conversion reaction without needing additional steam injection.

Description

Title of the Invention
Method and Apparatus for the Direct Reduction of Iron Ores utilizing Coal-Derived Gas or Syngas, with Improved Energy Efficiency.
Field of the Invention
The invention relates to the direct reduction of iron ores in a reduction system comprising a direct reduction process for producing direct-reduced-iron (DRI) utilizing a coal-derived gas or a syngas having a high CO content and a low ratio of H2 /CO, wherein the gas composition is modified by CO conversion to H2 and C02 followed by H20 and C02 removal (using the water- gas-shift reaction). The raw coal-derived gas may be produced by a variety of processes such as by partial oxidation, pyro lysis or gasification of hydrocarbons, for example coal gasifiers, coal melter-gasifiers, coke ovens, or even partial oxidation of gaseous or liquid fuels and the like (syngas). More particularly, the invention relates to a process and system to produce DRI with an improved energy efficiency and achieves this because more heat of the hot top gas effluent from the reduction reactor is recuperated by means of a liquid water circuit, as compared to using the heat only for steam generation. In one embodiment of the present invention, liquid water is heated by said hot gas withdrawn from the reduction reactor and the heated water is contacted with the coal-derived gas or syngas to provide all the H20 necessary for the CO conversion reaction.
Background of the Invention
Direct reduction of iron ores for producing pre-reduced metallized materials useful for the production of steel is becoming more and more widespread in the steel industry. Some of the advantages of direct reduction plants are that the production capacity may be relatively small as compared with pig iron production in coke-fed blast furnaces and that the metallic iron is produced in solid form with low sulfur and silicon content and that the DRI may be easily melted in electric-arc furnaces. The reducing agents utilized in the direct reduction plants are hydrogen and carbon monoxide, most typically produced by reformation of natural gas and therefore, these plants have been built in areas where natural gas is available and at relatively low price.
Hydrogen and carbon monoxide can also be produced by a variety of processes as partial oxidation, pyrolysis or gasification of hydrocarbons, for example coal gasifiers, coal melter- gasifiers, coke ovens, partial oxidation of gaseous or liquid fuels and the like. Coal-derived gas or syngas however typically has a low ratio of H2/CO as measured in volume, and therefore a high proportion of CO as compared with the traditional reformed gas produced by catalytic reformation of natural gas with H20 and C02.
There are a number of proposed processes and systems for treating raw syngas with low ratios of H2/CO before its utilization for direct reduction of iron ores, comprising conversion of CO with ¾0 to C02 and H2 by the following reaction, known as the water-gas-shift reaction: CO + H20 CO2 + H2
The above reaction is exothermic and takes place at moderate temperatures in the order of 300 to 600°C. The most efficient systems for carrying out this chemical CO conversion (shift) use two catalytic vessels with intermediate gas cooling of the partially shifted-gas.
The water necessary for the reaction is typically added to the raw syngas as steam. The steam can be produced in a dedicated boiler using a suitable fuel to produce the required amount of steam as described in the following prior-art patent publications:
US Patent No. 5,882,579 to Viramontes-Brown et al. describes a method and apparatus for producing DRI using syngas produced in a melter-gasifier and two reduction reactors. The gas effluent from the first reduction reactor associated to the melter-gasifier is treated to modify its CO content in at least one catalytic reactor (shifter) and the C02 resulting from the shift reaction and also the C02 from the reduction reactions in said second reduction reactor is removed from a combined stream of shifted-gas and recycled top gas effluent from the second reduction reactor. The water needed for the CO conversion is produced utilizing the heat of the top gas effluent from the second reactor to produce steam. Although this patent teaches using the heat of the top gas effluent from the reduction reactor to produce steam, it does not teach or suggest recuperating said heat of the top gas using liquid water as in the present invention.
US Patent No. 8,709,131 to Meissner et al. describes a method and system to produce DRI using synthesis gas, which may be generated in coal gasification processes, wherein the top gas withdrawn from the reduction reactor is treated in a CO shift catalytic reactor and then it is combined with raw synthesis gas before C02 is removed from the combined gas stream. The resulting gas stream is heated in two steps: a first step in a convection heater and a second step by partial combustion with oxygen, and is then fed to the reduction reactor. This patent however does not teach or suggest using the heat of the top gas to provide the water necessary for the CO shift reaction. This patent teaches using steam from an external source of the system and therefore the energy efficiency of the reduction system is lower as in the present invention. The heat of the reducing gas effluent from the reduction reactor is not used but wasted.
US Patent No. 5,958,107 to Greenwalt discloses a combination of a melter-gasifier unit 12 where the excess reducing gas effluent from a first reduction reactor 10 is subject to a CO conversion step to modify the content of carbon monoxide in said reducing gas before it is further utilized in a second direct reduction reactor 44 to produce DRI. This patent teaches that the water necessary to carry out the shift reaction of CO is provided in two ways: 50% by saturation of the gas stream in a saturator 24 where liquid water is sprayed over the reducing gas; and 50% by feeding steam 26 to the shift reactor. The steam may be produced using water condensed from a gas cooler after the shift reaction in a boiler 34. The heat to produce the steam 26 is an additional energy input to the plant.
The process of this patent is not as thermally efficient as the invention because it requires producing steam and consequently needs additional energy from a source external to the system. In contrast, the present invention uses as much heat as possible of the hot gas effluent from the reduction reactor, in the form of heated liquid water, to saturate the gas fed to the CO shift reactor and thus provide all the water necessary for the CO conversion. This is possible because the heat transfer coefficient between liquid water and a gas is higher than between two gas streams. The process of Greenwalt is also inefficient in the use of the reducing gases because the top gas effluent from the reduction reactor 44 is not upgraded by removal of C02 which is a product of the reduction reactions, therefore, the amount of top gas that can be recycled is limited and a good portion of the top gas has to be used as fuel to get rid of the C02 generated in the reactor 44. The present invention overcomes this disadvantage by combining the recycled gas from the reduction reactor with the gas "shifted" make-up gas stream and the resulting combined gas stream passes through the C02 removal unit thus allowing recycling almost all available reducing gases present in the top gas effluent from the reduction reactor.
U.S. Patent Application No. 2014/0202285 Al describes a method and system for treating waste gases from plants for pig iron production where said gases are utilized in a direct reduction plant. This patent application teaches a method of obtaining a gas stream with a desired ratio H2/CO between 1.5 to 4.5 starting from a syngas stream having a low ratio ¾/CO for example of 0.8, by mixing two gas streams: a first stream which has been subjected to CO conversion and C02 removal having a high ratio H2/CO for example of 16 and a second stream not having been subjected to CO conversion, only having been stripped of C02 and therefore having the same low ratio H2/CO for example of 0.8 of the untreated initial gas stream. This patent application however does not teach or suggest establishing a liquid water circuit for heat recuperation from the top gas effluent from the reduction reactor with higher efficiency. The heat content of the top gas 46 exiting the reduction reactor 18 is not used but wasted in cooler 49.
This patent application leads away of using liquid water for supplying all the water for the CO conversion and teaches that steam may be produced from other hot gases in the pig-iron production plant. The system of this patent application also has the disadvantage of requiring two separate C02 separation units in order to produce two gas streams with different composition which will be mixed to obtain the desired composition for iron ore reduction. The system of this publication has another drawback related to the introduction of a portion of the gas stream with high CO content into the transition zone of the reduction reactor with the purpose of increasing the carbon content of the DRI. It is known that carbon deposition from CO is not favored at high temperatures and at low temperatures so the carbon content in DRI is relatively low. In the present invention, the carbon content in DRI can be controlled in a wide range from about 2 % to about 4% by regulating the concentration of hydrocarbons in the reduction reactor. The carbon deposition is achieved by hydrocarbons cracking which is more effective at the process conditions of the transition zone or the cooling zone of the reduction reactor.
According to the invention, in contrast with the known practice of wasting the heat content of the top gas exiting the reduction reactor or preheating the recycle reducing gas by using available energy from the top gas in a heat exchanger or alternatively using this heat for low-pressure steam generation, instead the heat content of the reducing gas effluent from the reduction reactor is used to increase the water content of the reducing gas prior to the shift reaction of CO by means of a liquid water circulation system, offering a number of previously unrecognized advantages as, for example, increasing the amount of recovered heat from the hot top gas effluent from a reduction reactor and not requiring high-quality water for steam generation.
Documents cited in this text (including the foregoing listed patents), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention. Objects of the Invention
It is therefore an object of the present invention to provide a thermally efficient process and apparatus for producing DRI utilizing a syngas or coal-derived gas having a high content of CO.
It is a another object of the invention to provide a method and apparatus for increasing the efficiency of energy utilization in a direct reduction system for producing DRI, wherein heat from the top gas effluent from a reduction reactor is used to preheat and saturate a combined gas stream of recycled top gas and a make-up coal-derived gas or syngas stream using a liquid water closed circuit, instead of utilizing said heat for steam production, whereby more heat of the top gas effluent from the reduction reactor is recuperated and used.
It is a further object of the invention to provide a direct reduction plant for producing DRI with improved efficiency, wherein the DRI plant is advantageously combined with an iron melter-gasifier plant allowing for the utilization of a direct reduction process in locations where natural gas is not readily available or its utilization for DRI production may not be economically attractive.
It is still another object of the invention to provide a method and apparatus for producing DRI wherein the DRI plant is advantageously combined with an iron melter-gasifier plant, with improved thermal efficiency capable of producing said DRI with a controlled amount of chemically combined carbon in the range from 1.5% to 4%.
Other objects of the invention will be evident to those skilled in the art, or will be pointed out in the detailed description of the invention.
Summary of the Invention
The objects of the invention are generally achieved by providing a method for producing DRI utilizing a synthesis gas containing a relatively high content of CO, with a ratio H2/CO lower than about 0.5, in a reduction system comprising a reduction reactor from which a hot stream of reducing gas is withdrawn as a top gas, a heat-exchanger wherein heat is taken from said hot top gas and transferred to a stream of liquid water; and a gas humidifier.
The objects of the invention can also be more particularly achieved by providing a method method for producing DRI with improved energy efficiency in a direct reduction plant wherein a make-up first gas stream of at least one of a coal-derived gas or a syngas or a top gas from a reduction reactor or an export gas from a metallurgical furnace is subjected to a water- gas-shift reaction to H2 and C02 by reacting CO with H20 in at least one catalytic reactor resulting in a shifted reducing gas stream, which is used as reducing gas in a direct reduction reactor; said process being characterized by withdrawing a hot second gas stream from said reduction reactor; passing said second hot gas stream through a heat exchanger wherein heat is transferred from said second hot gas stream to a liquid water stream resulting in a heated liquid water stream; contacting at least a portion of said first gas stream with said heated liquid water stream in a humidifier to increase the water content of said first gas stream, thus providing all the H20 necessary for said water-gas-shift reaction, resulting in a water saturated third gas stream; passing said third gas stream through said catalytic reactor and through a C0 removal unit resulting in a fourth gas stream of reducing gas with an adequate composition for its use in the reduction process for producing DRI; and feeding said fourth gas stream to said reduction reactor.
The objects of the invention are also generally achieved by providing an apparatus for producing DRI utilizing a synthesis gas containing a high relative content of CO, with a ratio H2/CO lower than about 1, in a reduction system comprising a reduction reactor having a reduction zone, a gas-water heat exchanger; first gas conduit means connecting the heat exchanger to said reduction zone; a water gas humidifier where at least a portion of a top gas stream effluent from said reduction zone is contacted with liquid water that has been heated in said heat exchanger with heat of said top gas stream effluent from said reduction zone; second gas conduit means connecting the heat exchanger with said gas humidifier; at least one CO conversion reactor; third gas conduit means connecting the gas humidifier with said CO conversion reactor; a C02 removal unit; and fourth gas conduit means connecting said CO conversion reactor with said C02 separation unit; liquid water conduit means connecting said heat exchanger with said gas humidifier to circulate heated liquid water from the heat exchanger to said humidifier and returning liquid water from said humidifier to the heat exchanger after having contacted said top gas stream effluent from the reduction zone; and pumping means to circulate liquid water through said humidifier and said heat exchanger. Brief Description of the Drawings
Figure 1 is a schematic process diagram of a direct reduction plant using a coal-derived gas or a syngas wherein the heat of the hot top gas effluent from the reduction reactor is recuperated in a gas-liquid heat exchanger by a liquid water circuit and the heated water is contacted with a stream of the coal-derived gas or syngas; whereby all the water needed to modify the composition of said coal-derived gas in a CO conversion step is supplied by saturation of said gas stream.
Figure 2 is a schematic process diagram of the combination of a melter-gasifier coal gasification plant and a direct reduction plant incorporating the liquid water circuit for heat recuperation, wherein said melter-gasifier plant comprises an associated direct reduction reactor and the excess gas exported from said associated direct reduction reactor is used to produce DRI in said direct reduction plant.
Figure 3 is a schematic process diagram of the combination of a melter-gasifier coal gasification plant and a direct reduction plant incorporating the liquid water circuit for heat recuperation, wherein hot syngas produced in the melter-gasifier plant is fed directly to the reduction reactor of said direct reduction plant, combined with hot recycled and upgraded reducing gas, without passing first through an associated direct reduction reactor.
Detailed Description of Preferred Embodiments of the Invention
In Figures 1, 2 and 3, the elements that are common in all three figures are designated by the same numerals and the hot water circuit has been represented in thicker lines as compared to the gas narrower lines to facilitate the identification of the heat recuperation circuit with liquid water. Referring to the attached Figure 1, a reduction plant 10 comprises a reduction reactor 12 having an upper reduction zone 14 and a lower discharge zone 16. Particulate solid iron ores 18 in the form of lumps or pellets are contacted in the reduction zone 14 with a high-temperature reducing gas from pipe 20 comprising hydrogen and carbon monoxide and thereby producing direct reduced iron (DRI) 22. The DRI is discharged from said reactor 12 through the lower discharge zone 16. Depending on the type of subsequent utilization of the DRI, it may be discharged hot or cold. If discharged at high temperature from said reactor 12, it can be subsequently briquetted for further storage and handling, or it can be hot-fed directly into a steel- making furnace. If cold DRI is to be produced, the lower discharge zone 16 of reactor 12 may optionally have means (not shown), well known in the art, for circulating a stream of cooling gas for cooling down said DRI to a temperature level below about 100°C before its discharge from said reactor. See for example the cooling/discharge zone 14 with an associated cooling gas loop shown in US patent no. 4,524, 030.
A stream 15 of a hydrocarbon-containing gas, for example methane, such as coke oven gas, natural gas or liquified petroleum gas (LPG), can advantageously be used for increasing the carbon content of the DRI. It is known that carbon, when incorporated into the DRI chemically combined as iron carbide (known also as cementite), renders important economic benefits to the steelmaker because such carbon releases energy in the steelmaking furnace and reduces both the requirements of electric energy as well as the melting time in the melt-shop, significantly increasing the productivity of the melt-shop plant. To this end, the hydrocarbon-containing gas 15 is injected into the transition zone of the reactor 12, below the reduction zone 14 so that the high temperature of the DRI flowing down from the reduction zone cracks the hydrocarbons into carbon and hydrogen, resulting in DRI with higher carbon content. The carbon content of the DRI can be controlled by regulating the amount of the hydrocarbon-containing gas introduced into the reduction reactor. If the reduction reactor is designed to produce cold DRI, then the hydrocarbon-containing gas can also be injected to the cooling gas circuit (not shown for simplicity) and the carbon content of the DRI can also be controlled by regulating the amount of hydrocarbon-containing gas fed to the cooling gas circuit.
Spent reducing gas exits from the reduction zone 14 at a temperature in the range from about 300°C to about 500°C via pipe 24, identified here as a second gas source, to be used for upgrading in a recycle circuit and return back to the reduction zone 14. Such recycle reducing gas initially passes through a heat exchanger 26 where its sensible heat is used to preheat a stream of liquid water which is used in a gas humidifier 30. The spent reducing gas effluent from the reduction zone 14 contains significant amounts of water and carbon dioxide produced as by-products from the reactions of hydrogen and carbon monoxide with the iron oxide content of the iron ore 18. The upgrading of the reducing gas effluent from reduction zone 14 begins in the cooler/scrubber 34, where the water produced by the hydrogen reduction reaction condenses and is extracted from the system through pipe 38 along with the cooling water 36.
A minor portion of the cleaned and dewatered spent gas is purged from the recycle circuit through pipe 40 having a pressure control valve 42, for pressure control of, and for maintaining a lowN2 concentration in the recycle circuit. The purged gas may be advantageously utilized as fuel in the burner (not shown) of the gas heater 58 and optionally, if needed, may also be supplemented with some suitable fuel gas. The remaining portion of the cleaned and dewatered reducing effluent gas is then transferred to compressor 48 through pipes 50 and 54, wherein its pressure is raised to a level suitable for its ultimate recycling to the reduction zone 14 of the reactor 12.
According to an exemplary embodiment of the invention, the recycled reducing gas stream 50 after being combined with make-up gas stream 52 of upgraded syngas forms the reducing gas stream 53 which, after compression by compressor 48 is treated in a C02 removal unit 56. In case the C02 removal system is of the chemical type using a liquid solution, typically of diethanolamines, to absorb the C02, then C02 57 is withdrawn from the reduction system.
If the C02 removal system is of the physical type using adsorbent materials (PSA or VPSA), or gas membranes, to adsorb the C02, then C02 57 is withdrawn from the reduction system mixed with other gases like hydrogen, CO and methane, which can be utilized as fuel in the gas heater 58.
The C02-lean reducing gas stream exiting from C02 removal unit 56 passes through pipe 60 to heater 58 where its temperature is raised to above 850°C, preferably above 950°C. The hot reducing gas exiting from heater 58 through pipe 62 may be thereafter combined with oxygen, or oxygen enriched air, injected through pipe 64 from source 66, to further raise its temperature in the range from 1000°C to 1050°C. The hot reducing gas flows then through pipe 20 and is fed to the reduction zone 14 of reduction reactor 12.
Coal-derived gas or syngas from a make-up gas source 68 having a high content of CO, produced for example from a coal gasifier or a melter-gasifier system or a coke plant, which has been cooled/washed and compressed to a pressure sufficient to be injected into the reduction system, flows as a first gas stream through pipe 70 and pipe 72 to a humidifier 30 where the gas stream is saturated with water.
The saturated make-up gas stream flows from humidifier 30 through pipes 96 and 82 to a first CO-conversion vessel 84 containing a bed of catalyst 86. Since the CO conversion reaction is exothermic, the syngas stream is passed through heat exchanger 88 to bring down its temperature to the order of 300°C to 350°C and is then passed through a second CO-conversion vessel 90 containing a second bed of catalyst 92 to increase the efficiency of the CO-conversion to H2 and C02.
The syngas make-up stream exiting the second CO-conversion vessel 90 flows through pipe 94 and is cooled down in heat-exchanger 80 wherein the heat developed by the exothermic reaction in catalytic vessel 90 is used to pre-heat the water-saturated gas stream 96 effluent from humidifier 30. A pipe 98 and flow control valve 100 provide a by-pass line for adjusting the water content of the syngas to the desired level to carry out the CO conversion, by means of a temperature sensor and a pressure sensor 102 which emits signals indicative of the values of temperature and pressure of the gas stream in pipe 96. These signals are used to determine the water content of the syngas exiting the humidifier 30. A suitable automatic controller uses the calculated value of water content of the gas stream in pipe 96 to emit a control signal to flow control valve 100 so as to increase or decrease the flow of gas bypassing humidifier 30 and in this way allow less or more gas in contact with the heated liquid water. Since the amount of heat recovered in heat exchanger 26 is substantially constant, then the water-content in the gas stream exiting the humidifier 30 can be regulated at the level necessary to efficiently carry out the CO- conversion reaction by allowing more or less gas pass through humidifier 30.
The combined make-up syngas stream of the shifted gas stream flowing through pipe 104 exiting from heat exchanger 80 and the syngas stream flowing through pipe 98 bypassing humidifier 30, flows through pipe 106 and is cooled in a direct contact gas cooler 108 to about ambient temperature and exits cooler 108 as make-up stream 52 to be combined with the recycled gas 50 after C02 removal in C02 removal unit 56 and the hot gas is fed to the reduction zone 14. A stream of oxygen or oxygen-enriched air from source 66 is optionally added via pipe 64 to the hot reducing gas stream which further increases its temperature and therefore increases the productivity of the reduction reactor.
The reducing gas entering the reduction zone 14 through pipe 20 preferably has a composition characterized by a ratio of H2/CO in the range from 1.5 to 4.0 in volume percent and a reduction potential (H2 + CO) / (¾ + CO + H20 + C02) in the range of 0.8 to close to 1.
According to an exemplary embodiment of the invention, a circuit of liquid water is established, circulating through heat-exchanger 26 and humidifier 30, by means of pipes 74, 76 and a water pump 78. This liquid water circuit allows for a more efficient heat recovery from the reducing gas effluent from the reduction zone 14 because the liquid water may take heat at lower temperature as compared to the heat exchangers used for steam production.
Water from source 110 flows through pipe 112 and is pre-heated in heat exchanger 114, exiting through pipe 116 and passing through another heat exchanger 88 where its temperature is raised to a range between 150°C and 180°C and then is used as make-up water adding it through pipe 118 to the liquid water circuit 120 comprising heat exchanger 26, humidifier 30, pipes 74 and 76 and pump 78.
A portion of the hot water circulating through the hot water circuit is withdrawn through pipe 128 provided with flow control valve 130 to purge some water (through pipe 132) to minimize concentration of sediments and dirt in the water circuit 120. This blow-down hot water stream is passed through heat exchanger 114 and contributes to preheat the make-up water stream from pipe 112.
A pipe 122 with a flow control valve 124 is provided to by-pass a controlled amount of hot liquid water in response to a control signal of a temperature sensor 126 measuring the temperature of the water returning to heat exchanger 26 through pipe 74.
This control loop of the temperature of the liquid water returning to heat exchanger 26 is used to prevent the water entering heat exchanger 26 from reaching too low a temperature that may cause condensation of the water contained in the top gas stream and cause problems of clogging and gas flow obstructions in heat exchanger 26. By way of example, the temperature of the water stream entering heat exchanger 26 is maintained above 120°C at a gas pressure of about 5 Kg/cm2A. The condensation temperature of water of course depends on the pressure of the gas and therefore the range of temperature for this control has to be adapted to the particular operation conditions of each application. When the water temperature in pipe 74 is lower than about 115°C, the valve 124 allows more hot water bypass the humidifier 30 raising in this way the water temperature.
Using a liquid water circuit for heat recuperation as in the present invention, provides the advantage of using about 30% more heat as compared with the currently used steam production heat exchangers. In an exemplary embodiment of the invention, the amount of heat recuperated when using a liquid water circuit is about 0.27 Gcal/ton of DRI while only about 0.21 Gcal/ton of DRI can be recuperated using a steam production heat exchanger. Another advantage of using liquid water instead of producing steam in heat-exchanger 26 is that the quality of water used is less demanding, thus providing operational and capital savings.
Referring to Figure 2, a schematic diagram of the liquid water heat recuperation circuit 120 is shown as applied to a combination of a melter-gasifier plant having a melter-gasifier 134, wherein fossil fuels such as coal, refinery residues and biomass are partially oxidized with oxygen 136 and steam 138 in a manner known in the art. The syngas exits through pipe 140, is de-dusted and partially cleaned in a hot cyclone 142, and is then fed to a reduction reactor 144, to which iron ore 146 is also fed to produce a metallized product which is discharged through a discharge system 148 and fed to the melter-gasifier 134, where the product is melted. Pig iron 150 and slag 152 are tapped from melter-gasifier 134 as known in the art.
After quenching and cleaning in gas cleaning system 154 comprising at least a gas scrubber, the composition of the syngas exiting via pipe 156 and is compressed by compressor 158 to be subjected via pipe 70 to the CO conversion step to increase the hydrogen content and obtain a H2/CO ratio of 2 to 3 (measured by % volume) as described above in connection with Figure 1.
With reference to Figure 3, a schematic diagram of the liquid water heat recuperation circuit 120 is shown as applied to a combination of a melter-gasifier plant having a melter- gasifier 134, wherein the hot syngas exiting through pipe 70, after de-dusting and partial cleaning in a hot cyclone 142, is directly fed to the reduction zone 14 combined with upgraded recycled gas effluent from the reduction zone 14.
The hot syngas stream 70 is at a high temperature (above 900°C) and is combined with hot recycled and upgraded gas which has been subjected to the CO conversion step as described in connection with Figure 1 to obtain the required composition about 50% of the reducing gas required in reduction zone 14. The rest of the process is the same as described in connection with Figure 1.
The process of the invention offers a number of advantages over the prior art. For example, it does not require steam production spending additional thermal energy for supplying the water used as reactant in the catalytic shift reactor. The hot water necessary for gas saturation is heated using waste energy from the gas effluent from the reduction reactor instead of being produced in external heaters or boilers. The energy efficiency of the DR process is particularly improved, since about 0.27 Gcal per ton of DRI is recuperated from the hot gas effluent from the reduction reactor as compared with other prior-art processes that produce steam with the heat of said effluent gas stream or require an additional boiler.
The higher thermal efficiency also comes from the fact that the heat transfer coefficient for a gas-liquid water heat exchanger is higher than for gas-gas heat exchanger.
Additionally, since no steam is produced in the heat exchanger, the temperature of the gas stream is lower as compared with the lowest temperature that can be reached for the gas stream and still produce steam.
The water circuit allows recuperation of almost all the heat available, because all the non- evaporated excess water withdrawn from the gas humidifier is re-heated in the heat exchanger without any heat waste.
Example:
A process calculation for a DRI plant having a production capacity of 220 metric tons of DRI per hour indicates the following:
A second gas stream of hot top gas 24 exits the reduction zone 14 of the DRI plant at a temperature of 484°C, a flow rate of 463,289 NCMH and a water content of 22.2% volume. This gas stream passes through a heat exchanger 26 (See figure 1) and exits through pipe 32 at a temperature of 130°C, whereby the major part of its sensible heat is transferred to a stream of liquid water 74 entering said heat exchanger at a temperature of 115°C and is heated to form a heated liquid water stream 76 at a temperature of 165°C. 1,166 tons per hour of the heated liquid water stream circulates to the humidifier 30 where it is contacted with a first gas stream of coal- derived gas from source 68 having a ratio H2/CO = 0.4 increasing its water content from 2% to 43% equivalent to about 137 tons per hour of water incorporated into said gas stream. This forms a third gas stream having an amount of water sufficient to carry out the shift conversion of CO in the catalytic reactors 84 and 90 so that the ratio H2 / CO is increased to 10, which is a value adequate for supplying the modified coal-derived gas or syngas to the reduction reactor system. The shift modified resulting fourth gas stream 94 is then eventually recycled back to the reduction zone 14, after being subsequently cooled down in cooler 108 whereby water is condensed and the water content in the gas stream is lowered from 18.4%) to 0.8%) and then being combined with the cooled top gas effluent 50 derived from the reduction zone 14. The combined gas stream 53 passes through a compressor 48 and is fed to a PSA - C02 separation unit 56 resulting in a reducing gas stream in pipe 60 having a high reduction potential measured as (H2 + CO) / (¾ + CO + H20 + C02) of 0.9 adequate for carrying out the reduction reactions in the reduction zone 14 and a ratio H2 / CO of 3.4 adequate for its handling in the reduction system without problems in the gas heater.
It is of course to be understood that in this specification only some preferred
embodiments of the invention have been described for illustration purposes and that the scope of the invention is not limited by such described embodiments but only by the scope of the appended claims.

Claims

What is claimed is:
1. A method for producing DRI with improved energy efficiency in a direct reduction plant wherein a make-up first gas stream of at least one of a coal-derived gas or a syngas or a top gas from a reduction reactor or an export gas from a metallurgical furnace is subjected to a water-gas-shift reaction to H2 and C02 by reacting CO with H20 in at least one catalytic reactor resulting in a shifted reducing gas stream, which is used as reducing gas in a direct reduction reactor; said process being characterized by
withdrawing a hot second gas stream from said reduction reactor;
passing said second hot gas stream through a heat exchanger wherein heat is transferred from said second hot gas stream to a liquid water stream resulting in a heated liquid water stream;
contacting at least a portion of said first gas stream with said heated liquid water stream in a humidifier to increase the water content of said first gas stream, thus providing all the H20 necessary for said water-gas-shift reaction, resulting in a water saturated third gas stream;
passing said third gas stream through said catalytic reactor and through a C02 removal unit resulting in a fourth gas stream of reducing gas with an adequate composition for its use in the reduction process for producing DRI; and
feeding said fourth gas stream to said reduction reactor.
2. A method for producing DRI according to claim 1, being further characterized by said first stream being a top gas effluent from a reduction reactor.
3. A method for producing DRI according to claim 1, being further characterized by combining at least a portion of the second gas stream after it exits from said heat exchanger with said fourth gas stream prior to feeding the combined gas stream through the C02 removal unit.
4. A method for producing DRI according to claim 1, being further characterized by performing the water-gas-shift reaction in two steps and cooling the gas effluent from the first water-gas-shift reaction step prior to feeding it to the water-gas-shift reaction step.
5. A method for producing DRI according to claim 1, being further characterized by splitting said first gas stream in two portions; a first portion passing through said humidifier and said catalytic reactor and a second portion by-passing said humidifier and said catalytic reactor, and combining said first portion and said second portion of the first gas stream before feeding the combined portions to the C02 removal unit.
6. A method for producing DRI according to claim 5, being further characterized by regulating the flow rate of said second portion of the first gas stream in response to a signal emitted by a controller based on a calculated value of the gas water content using the pressure and temperature of the gas stream exiting said humidifier.
7. A method for producing DRI according to claim 1, being further characterized by splitting said heated liquid water stream in two portions; a first portion passing through said humidifier and a second portion by-passing said humidifier, and combining said first portion and said second portion of the heated water stream beyond the humidifier but before being returned to said heat exchanger.
8. A method for producing DRI according to claim 7, being further characterized by regulating the flow rate of said second portion of the heated liquid water stream in response to a calculated value of the gas water content using the pressure and temperature of the recombined first and second heated liquid water stream portions.
9. A method for producing DRI according to claim 1, being further characterized by said first stream being a top gas effluent from a reduction reactor.
10. A process according to claim 1, further characterized by said coal-derived gas is coke oven gas.
11. A process according to claim 1, further characterized by injecting a stream of at least one of coke oven gas, natural gas or liquefied petroleum gas at the transition zone of said reduction reactor to increase the carbon content of the DRI.
12. A process according to claim 1, further characterized by said reduction reactor comprising a cooling zone where a cooling gas is circulated to cool the DRI to a temperature below about 100°C and injecting a stream of at least one of coke oven gas, natural gas or liquefied petroleum gas to said reduction reactor along with said cooling gas to increase the carbon content of the DRI.
13. Apparatus for producing DRI utilizing a coal-derived gas or a syngas containing a high relative content of CO, in a reduction system comprising
a reduction reactor having a reduction zone,
a gas-water heat exchanger;
first gas conduit means connecting the gas-water heat exchanger to said reduction zone; a water gas humidifier for contacting at least a portion of a top gas stream effluent from said reduction zone with liquid water heated in said gas-water heat exchanger with heat of said top gas stream effluent from said reduction zone;
second gas conduit means connecting the water gas humidifier with the catalytic reactor; at least one catalytic reactor for a water-gas-shift reaction;
third gas conduit means connecting the water gas humidifier with said catalytic reactor; a C02 removal unit; and
fourth gas conduit means connecting said catalytic reactor with said C02 removal unit; liquid water conduit means connecting said gas-water heat exchanger with said water gas humidifier for circulating heated liquid water from the gas-water heat exchanger to said water gas humidifier and for returning liquid water from said water gas humidifier to the gas-water heat exchanger after having contacted said top gas stream effluent from the reduction zone; and pumping means for circulating liquid water through said water gas humidifier and said gas-water heat exchanger.
14. Apparatus for producing DRI according to claim 13, being further characterized by comprising
fifth conduit means for accessing a source of said coal-derived gas or syngas and for delivering a first portion to pass through said second conduit means and a second portion to bypass said humidifier and said catalytic reactor, and
gas mixing means for combining said first portion and said second portion before the C02 removal unit.
15. Apparatus for producing DRI according to claim 14, being further characterized by comprising
a controller capable calculating the value of the gas water content using the pressure and temperature of the gas stream exiting said humidifier and
flow regulating means to control the flow rate of said second portion of the first gas stream in response to a signal emitted by said controller.
16. Apparatus for producing DRI according to claim 13, being further characterized by comprising
second water conduit means for splitting said heated liquid water into two portions; a first heated liquid water portion to pass said heated liquid water on through said water conduit means to said humidifier and
a second heated liquid water portion to pass said heated liquid water on through said second water conduit means to bypass said humidifier, and
water mixing means for combining said first portion and said second portion of the heated liquid water protions before returning heated liquid water to said heat exchanger.
17. Apparatus for producing DRI according to claim 16, being further characterized by comprising
a controller for regulating the flow rate of said second portion of the heated liquid water in response to a signal indicative of a calculated value of the gas water content and a sensor capable of emitting control signals indicative of the pressure and temperature of the gas stream exiting said humidifier to calculate said gas water content.
18. A process according to claim 13, further characterized by comprising
gas injection means to introduce a stream of at least one of coke oven gas, natural gas or liquefied petroleum gas at the transition zone of said reduction reactor to increase the carbon content of the DRI.
19. A process according to claim 13, further characterized by said reduction reactor comprising
a cooling zone where a cooling gas is circulated to cool the DRI to a temperature below about 100°C and gas injection means to introduce a stream of at least one of coke oven gas, natural gas or liquefied petroleum gas to said reduction reactor along with said cooling gas to increase the carbon content of the DRI.
PCT/IB2016/001444 2015-09-16 2016-09-16 Method and apparatus for the direct reduction of iron ores utilizing coal-derived gas or syngas, with improved energy efficiency WO2017046653A1 (en)

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